Will Young, director of Zappos Labs, told Bloomberg that Pinterest users are far more likely to share a purchase than Twitter or Facebook users—but that shared items generate far less revenue than Twitter or Facebook.

This is a big problem for Pinterest, because the whole idea of the site is that it's supposed to be better at monetizing social activity than Twitter or Facebook.

Young told Bloomberg that Zappos customers were 13 times likelier to share an item they bought with friends on Pinterest than on Twitter, and 8 times likelier than on Facebook.

But a post on Twitter generated far more revenue—$33.66 an order—than Facebook, at $2.08 an order, or Pinterest, at 75 cents an order.

That's great news for Twitter, which will surely tout these figures as it makes a push for retail advertisers.

But it's kind of bad news for Pinterest, which recently raised $100 million at a $1.5 billion valuation on the premise that its site—which is all about sharing beautiful images of things to buy—should be good at this kind of social commerce.

It's not great news for Facebook, either, which has ambitions to make commerce a bigger part of the site than it is today.

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Will Young from Zappos Labs here. I wanted to clarify that we're huge fans of Pinterest. We love what they do and it's why we built Pinpointing (even already knowing some of the order share data). Zappos Labs is really focused and sees value on many social platforms like Pinterest, Twitter, and Facebook. We're constantly trying to learn how our customers want to interact with different channels and create the best experience possible for them. We think our customers who use Pinterest (and there are a lot) are going to love to try this different way to shop Zappos.